Ultrasonic nondestructive techniques are well known as a tool for evaluating and analyzing physical characteristics and properties of bodies. Such known ultrasonic inspecting techniques use an ultrasonic transducer coupled to the body under test for transmitting ultrasonic signals thereinto. Any internal defect, crack, or other discontinuity within the body will reflect a portion of the ultrasonic signals back toward a receiving transducer. The time interval between the launching of the ultrasonic signals into the body and the reception of the reflected signals indicates the location of an acoustic impedance change within the body.
In order to achieve a uniform acoustical coupling between the transducer and the body under test, it has been suggested to acoustically couple the transducer to the body by means of a suitable coupling fluid. One such known technique, which requires immersion of the body and the transducer in the coupling fluid does effect uniform application of the signals into and out of the body but does not have repeatable ultrasonic transmission coefficients between the transducer and the body. It is often impractical, if not impossible, to immerse the body under test especially when such a body is an intrinsic part of a somewhat larger apparatus or arrangement.
Instead of using immersion, coupling to the body under test can be achieved by using a thin layer of a bonding agent to improve the transmission coefficient at the transducer/body interface. The bonding agent may permanently affix the transducer to the body, or it may temporarily provide coupling between the transducer and the body under test. The coupling resulting from such a thin bonding layer, is generally not accurately reproducible. Furthermore, such coupling, which is usually unmeasurable, leads to ambiguities in the values of the transmission coefficient and in the interpretation of returning echoes.
Therefore, there exists a need for a reliable and reproducible nondestructive method for ultrasonically inspecting certain characteristics of a body independently of the transmission coefficient between the transducer and the body for any given measurement.